1. Field of the Invention
The present invention relates to a liquid crystal display device used for a display part of an information equipment or the like, and a driving method of the same.
2. Description of the Related Art
In recent years, a liquid crystal display device has been improved so as to have a large size, high gradation, and high contrast, and has been used for a monitor of a PC (Personal Computer) or a television receiver or the like. In these uses, such excellent viewing angle characteristics that a display screen can be seen in any directions are required.
Since a color liquid crystal display device is yet inferior to a CRT (Cathode-Ray Tube) in the viewing angle characteristics, the realization of a wide viewing angle is desired. As a method of widening the viewing angle of the liquid crystal display device, there is an MVA (Multi-domain Vertical Alignment) mode. FIGS. 27A and 27B show the schematic sectional construction of a liquid crystal display device of the MVA mode. FIG. 27A shows a state where a voltage is not applied to a liquid crystal layer, and FIG. 27B shows a state where a predetermined voltage is applied to the liquid crystal layer. As shown in FIGS. 27A and 27B, the liquid crystal display device includes substrates 302 and 304 disposed to be opposite to each other. Transparent electrodes (not shown) are formed on both the substrates 302 and 304. Besides, plural linear protrusions 306 parallel to each other are formed on the one substrate 302, and plural linear protrusions 308 parallel to each other are formed on the other substrate 304. The protrusions 306 and 308 are alternately arranged when viewed in a direction vertical to a substrate surface.
A liquid crystal layer 160 having a negative dielectric anisotropy is sealed between both the substrates 302 and 304. As shown in FIG. 27A, liquid crystal molecules 312 are aligned almost vertically to the substrate surface by the alignment regulating force of vertical alignment films (not shown) formed on opposite surfaces of both the substrates 302 and 304. The liquid crystal molecules 312 in the vicinity of the protrusions 306 and 308 are aligned almost vertically to oblique surfaces formed by the protrusions 306 and 308. That is, the liquid crystal molecules 312 in the vicinity of the protrusions 306 and 308 are aligned obliquely with respect to the substrate surfaces.
As shown in FIG. 27B, when the predetermined voltage is applied between the transparent electrodes of both the substrates 302 and 304, the liquid crystal molecules 312 in the vicinity of the protrusions 306 and 308 are inclined in the directions vertical to the extending directions of the protrusions 306 and 308. The inclination is propagated to the respective liquid crystal molecules 312 between the protrusions 306 and 308, and the liquid crystal molecules 312 in a region between the protrusions 306 and 308 are inclined in the same direction.
As stated above, by disposing alignment regulating structures such as the protrusions 306 and 308, inclination directions of the liquid crystal molecules 312 can be regulated for each region. When the alignment regulating structures are formed in two directions almost vertical to each other, the liquid crystal molecules 312 are inclined in four directions in one pixel. As a result of the mixture of viewing angle characteristics of the respective regions, a wide viewing angle in a white display or a black display can be obtained. In the liquid crystal display device of the MVA mode, ten or more contrast ratios are obtained even at an angle of 80° or more in vertical and horizontal directions from a direction vertical to a display screen.
In the liquid crystal display device of the MVA mode, a vertical alignment technique to realize high contrast and high speed response and an alignment dividing technique to realize a wide viewing angle are combined and used. In the alignment dividing technique, the alignment regulating structures, such as the linear protrusions 306 and 308 or electrode removal parts (slits), are formed on the substrates. Since the alignment directions of the liquid crystal molecules 312 are regulated by these alignment regulating structures, and a rubbing treatment which becomes the great cause of a drop in productivity becomes unnecessary, high productivity is realized.
Besides, in order to realize the liquid crystal display device of the MVA mode having higher display quality, there is a technique in which a photo-cured material is formed in a liquid crystal layer 160 so that the alignment regulating force of the liquid crystal molecules 312 is increased. A liquid crystal containing a photo-curing composition (resin) is injected in a liquid crystal display panel, and the photo-cured material is formed in a state where a voltage is applied, so that a predetermined pre-tilt angle can be given to the whole of each of aligned regions divided by the alignment regulating structures. By this, alignment abnormal regions of the liquid crystal molecules 312 are decreased and high transmission factor can be realized, and further, since the propagation of inclination of the liquid crystal molecules 312 becomes almost unnecessary, a high speed response can be realized.
As the alignment regulating structures, in addition to the protrusions 306 and 308 and the slits, there is also a minute electrode pattern. FIG. 28 shows one pixel in which the minute electrode pattern is formed. As shown in FIG. 28, plural gate bus lines 104 (only one line is shown in FIG. 28) extending in the horizontal direction in the drawing, and plural drain bus lines 106 (two lines are shown in the drawing) intersecting with the gate bus lines 104 through a not-shown insulating film and extending in the vertical direction in the drawing are formed on a TFT substrate 102. A TFT 110 is formed in the vicinity of an intersecting position of the gate bus line 104 and the drain bus line 106. Besides, a storage capacitor bus line 108 is formed to cross substantially the center of a rectangular pixel region defined by the gate bus line 104 and the drain bus line 106.
Cross-shaped connection electrodes 120 and 122 are formed in the rectangular pixel region to divide it in four rectangles of the same shape. The connection electrode 122 is formed to cross the center of the pixel region and to be parallel to the drain bus line 106, and the connection electrode 120 is formed on the storage capacitor bus line 108. Besides, plural stripe electrodes 124 are formed which extend from the connection electrodes 120 and 122 at an angle of 45° and form the minute electrode pattern. A space 126 in a state where an electrode is removed is formed between the adjacent stripe electrodes 124. A pixel electrode is constituted by the connection electrodes 120 and 122, the plural stripe electrodes 124 and the spaces 126. Besides, alignment regulating structures are constituted by the stripe electrodes 124 and the spaces 126. Each of the stripe electrodes 124 is formed to have a width L1, and each of the spaces 126 is formed to have a width S1.
FIGS. 29 and 30 show a section of the liquid crystal display device taken along line B-B of FIG. 28. FIG. 29 shows a state where a voltage is not applied to the liquid crystal layer 160, and FIG. 30 shows a state where a voltage is applied to the liquid crystal layer 160. As shown in FIGS. 29 and 30, the TFT substrate 102 includes the stripe electrodes 124 on a glass substrate 150. An opposite substrate 103 disposed to be opposite to the TFT substrate 102 includes a common electrode 154 on a glass substrate 151. Vertical alignment films 152 and 153 are formed on surfaces of the TFT substrate 102 and the opposite substrate 103 in contact with the liquid crystal layer 160, respectively.
In the state where the voltage is not applied to the liquid crystal layer 160, as shown in FIG. 29, the liquid crystal molecules 312 are aligned almost vertically to the substrate surface. In the state where the voltage is applied to the liquid crystal layer 160, as shown in FIG. 30, the liquid crystal molecules 312 are inclined toward the connection electrodes 122 and 124 in the extending directions of the stripe electrodes 124, and are aligned almost parallel to the substrate surface.
Also by the construction shown in FIG. 28, by dividing the alignment directions of the liquid crystal molecules 312 in quarters in one pixel, a wide viewing angle is obtained in a white display or a black display. However, since the alignment regulating force of the liquid crystal molecules 312 caused by only the minute electrode pattern is low, similarly to the above, a photo-cured material is formed in the liquid crystal layer 160 and the alignment regulating force is increased. The photo-cured material is formed in such a way that a photo-curing composition (monomer) capable of being polymerized by light is mixed in the liquid crystal layer 160, and irradiated by light such as ultraviolet rays (UV) in a state where a predetermined voltage is applied.
FIG. 31 is a graph showing transmission characteristics (T-V characteristics) of the MVA mode liquid crystal display device. The horizontal axis indicates applied voltage (V) to the liquid crystal layer 160, and the vertical axis indicates transmission factor (%) of light. A curved line A expressed by a solid line in the graph indicates a T-V characteristic in a direction vertical to a display screen (hereinafter referred to as “front direction”), and a curved line B expressed by a solid line plotted by Δ marks indicates a T-V characteristic in a direction of an azimuth angle of 90° and a polar angle of 60° (hereinafter referred to as “oblique direction”). Here, the azimuth angle is an angle measured in the counterclockwise direction from almost the center of the display screen on the basis of the horizontal direction. The polar angle is an angle with respect to the vertical line drawn at the center of the display screen. The display mode of the liquid crystal display device is a normally black mode in which an applied voltage to the liquid crystal layer 160 is lowered to display black, and an applied voltage is raised to display white. It is desirable that the T-V characteristics are constant independent of the viewing angle.
However, as shown in FIG. 31, the curved line A indicating the T-V characteristic in the front direction intersects the curved line B indicating the T-V characteristic at a point in the vicinity of an applied voltage of about 2.7 V. The transmission factor in the oblique direction is higher than the transmission factor in the front direction at an applied voltage of 2.7 V or less, and is lower than the transmission factor in the front direction at an applied voltage of 2.7 V or higher. Thus, in the range of the applied voltage of from 1.5 V to 2.7 V, since the transmission factor in the oblique direction is higher than that in the front direction, there arises a problem that when viewed in the oblique direction, the display image is seen to be whitish. Besides, since the transmission factor in the oblique direction is lower than the transmission factor in the front direction in the range of a relatively high applied voltage, when viewed in the oblique direction, the contrast in the whole display screen is lowered.
The transmission factor is varied in accordance with the retardation (Δn·d) of the liquid crystal layer 160. When viewed in the oblique direction, since the substantial retardation of the liquid crystal layer 160 is lessened by the liquid crystal molecules 312 inclined in the oblique direction, the above problem arises. Besides, also with respect to chromaticity, since the weight of the transmission factor from each pixel is changed between a case where it is seen in the front direction and a case where it is seen in the oblique direction, there arises a problem that the chromaticity is changed.
FIG. 32 is a graph showing the T-V characteristics when the display screen of the MVA mode liquid crystal display device is observed at plural viewing angles. The horizontal axis indicates the applied voltage (V) to the liquid crystal layer 160, and the vertical axis indicates the transmission factor (%) of light. A curved line A in the graph indicates the T-V characteristic in the front direction. Curved lines B, C, D and E indicate the T-V characteristics in the directions of an azimuth angle of 90° and polar angles of 20°, 40°, 60° and 80°, respectively. As shown in FIG. 32, an undulation occurs on the curved line E in a region F, and there is a range in which even if the applied voltage is raised, the transmission factor is lowered. Thus, there arises a problem that a display image is reversed between the front direction and the direction of a polar angle of 80°.